Reproduction steps:

• Go to site administration->Languages->Language packs
• Select two languages that have parent language packs but the parent language packs are not yet installed; do not select parent packs. For example, select "es_wp" and "de_wp" (make sure that "es" and "de" are neither installed nor selected)
• Push button to install languages
• You will see that three languages are installed: "es_wp", "de_wp" and "de". Language "es" is not installed

If you reload the same page you will see an error message that language "es" is not installed.

This happens because of the list of translations is cached after the first language is installed and after the second language is installed it can not be found and the parent of it can not be found.

If $CFG->langlist is set, the parent would not be installed even for one language.

We find it very annoying on workplacedemo site where we regularly install multiple workplace language packs.